Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
J Chem Theory Comput. 2021 Dec 14;17(12):7428-7446. doi: 10.1021/acs.jctc.1c00402. Epub 2021 Nov 24.
We present the theory and implementation of a core-valence separated similarity transformed EOM-CCSD (STEOM-CCSD) method for K-edge core excitation spectra. The method can select an appropriate active space using CIS natural orbitals and near "black box" to use. The second similarity transformed Hamiltonian is diagonalized in the space of single excitation. Therefore, the final diagonalization step is free from the convergence problem arising due to the coupling of the core-excited states with the continuum of doubly excited states. Convergence trouble can appear for the preceding core-ionized state calculation in STEOM-CCSD. A core-valence separation (CVS) scheme compatible with the natural orbital based active space selection (CVS-STEOM-CCSD-NO) is implemented to overcome the problem. The CVS-STEOM-CCSD-NO has a similar accuracy to that of the standard CVS-EOM-CCSD method but comes with a lower computational cost. The modification required in the CVS scheme to make use of the CIS natural orbital is highlighted. The suitability of the CVS-STEOM-CCSD-NO method for chemical application is demonstrated by simulating the K-edge spectra of glycine and thymine.
我们提出了一种用于 K 边芯激发谱的核价分离相似变换 EOM-CCSD（STEOM-CCSD）方法的理论和实现。该方法可以使用 CIS 自然轨道选择合适的活性空间，并近乎“黑箱”使用。第二个相似变换哈密顿量在单激发空间中对角化。因此，最终的对角化步骤不受由于芯激发态与双激发态连续体耦合引起的收敛问题的影响。在 STEOM-CCSD 中，前芯电离态计算可能会出现收敛问题。实现了与基于自然轨道的活性空间选择兼容的核价分离（CVS）方案（CVS-STEOM-CCSD-NO）来克服该问题。CVS-STEOM-CCSD-NO 具有与标准 CVS-EOM-CCSD 方法相似的准确性，但计算成本更低。突出显示了在 CVS 方案中进行的利用 CIS 自然轨道的修改。通过模拟甘氨酸和胸腺嘧啶的 K 边光谱，证明了 CVS-STEOM-CCSD-NO 方法在化学应用中的适用性。
